Centrifugal pump design and analysis
AIM
To design and analyze the flow through centrifugal pump
OBJECTIVE
- Model the centrifugal pump
- Obtain the relationship between Pressure ratio and mass flow rate and explain your results.
where Pressure ratio = Outlet total pressure/ Inlet total pressure
THEORY
Centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy with the help of impellers. Fluid enters into the fast rotating impeller along its axis and is thrown out by centrifugal force along its circumference through the impeller’s vane tips. The action of the impeller increases the kinetic energy of the fluid and pressure and also directs it towards the pump outlet. The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge.
WORKING
The impeller is the key component of a centrifugal pump. It consists of a series of curved vanes. These are normally sandwiched between two discs. Fluid enters the impeller along its axis and exits along the circumference of the impeller. The impeller on the other side is connected to a drive shaft and to a motor which is rotated at high speed. The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing.
In comparison to the positive displacement pumps, centrifugal pumps are usually specified for higher flowrates. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, both the pumps have their own advantages and disadvantges.
Centrifugal pumps are commonly used for pumping water, solvents, organics, oils, acids, bases and any thin liquids in both industrial, agricultural and domestic applications. The design of centrifugal pump is most suitable for application involving low viscosity fluids.
The efficient operation of a centrifugal pump relies on the constant, high speed rotation of its impeller. With high viscosity feeds, centrifugal pumps become inefficient, the resistance will be higher, so high pressure is needed to maintain a specific flow rate.
FLOW SETUP
GEOMETRIC MODEL
Inputs Parameters
Analysis Type: Internal
Type of fluid: Water
Type of flow: Laminar and Turbulant
Wall conditions: Adiabatic with 0 micrometer roughness,
Initial conditions: Total Pressure(101325Pa), Temperature(293.2K).
Boundary conditions
@INLET Total pressure of 101325Pa
@OUTLET outlet velocity of 10m/s
Now the parametric study is carried out for different outlet velocity, the total pressure at the outlet is found.
CASE 1 @ 10m/s outlet velocity
CASE 2 @ 20m/s outlet velocity
CASE 3 @ 30m/s outlet velocity
CASE 4 @ 40m/s outlet velocity
CASE 5 @ 50m/s outlet velocity
TABLE OF TOTAL PRESSURE AND MASS FLOW RATE AT DIFFERENT OUTLET VELOCITIES
Inlet total pressure is 101325 pa for all the cases
Calculation of pressure ratio
@10m/s 2.17
@20m/s 1.3
@30m/s 7.02
@40m/s 18.77
@50m/s 32.14
The graph of mass flow rate vs pressure ratio is plotted which is shown below
CONCLUSIONS
- As the outlet velocity increases the mass flow rate also increases at the outlet.
- If we compare the theoritical plot of mass flow rate vs pressure ratio we have a parabolic profile but our graph is not that accurate as we did not use much refined mesh, so approximate result is obtained.
- From the performance diagram between Pressure ratio and mass flow rate, it can be concluded that as the pressure ratio increases, mass flow rate of water coming out from outet also increases.
- At one point we obtain the peak point of pressure ratio and again with increase in mass flow rate, the pressure ratio decreases.